I'm no expert but here is what I was told when my youngest had it a few months back...it's contagious for about 10 days (I can't remember if she said after the rash appears or just whenever diagnosed) and that the stools are contagious for a month. She also told me that adults don't usually get it (though she herself did once), that it's usually kids and once they get it they generally don't get it again, though other people have said there are different strands of it, so there are some discrepancies.

Contagious while fever is present until rash appears..I'm sure the 5 mo will get it. Mom, probably not. I wouldn't be around him until it is all clear. Ds had it last summer , he also has had roseola twice now. Dh did get the roseola the 2 nd time, both had fevers and bad rash. With HFM ds had high fever, sore throat w white spots, and a few spots/blisters on hands and soles of feet.

__________________
My name is Amanda, happily married to Ernesto I'm a former HS teacher turned SAHM to Jake 6.7.10 and Norah 7.23.12

When my kids got it, we were told they were contagious for the week prior to any symptoms showing (the incubation period) and during the fever stage (in our house it lasted 3 days) - once the rash appeared they were no longer contagious. So based on that, I would avoid your friends for a while until the bug had worked itself out of their house completely. I'm pretty sure that adults can be carriers of it to and pass it on to the kids even if they themselves haven't caught it.